Attempts have been made in the prior art to develop an antithrombin agent as an antithrombotic agent. However, it has been known that such an antithrombin agent is apt to cause bleeding tendency and difficulty to manage haemostasis because it inhibits blood coagulation and also thrombin induced platelet aggregation. With the aim of overcoming such problems, the development of anticoagulant agents has been attempted based on a inhibitory mechanism other than the thrombin inhibition. As a result of such efforts, 1,2-bis(5-amidino-2-benzofuranyl)ethane (hereinafter, referred to as "DABE") represented by the following formula (2) has been found as an anticoagulant agent based on FXa inhibition (Thrombosis Research, vol.19, pp. 339-349, 1980): ##STR3##
However, DABE has the disadvantages that it has both FXa and thrombin inhibitory activities which cannot be separated sufficiently from each other, it has a very low water solubility, and it does not show its anticoagulant effect when administered orally. Consequently, great attention has been directed, from a clinical point of view, toward the development of a drug which is highly specific and potent inhibitor for FXa, has a high water solubility, and is effective in oral administration.